1. Field of the Invention
This invention relates in general to ultrasonic imaging. The invention is particularly directed to linear scanning type ultrasonic imaging devices, and more specifically is directed to such a device in which the enlargement and contraction of an image are effected through combined write and read zoom operations at the predetermined magnification factor. This invention includes both apparatus for and methods of ultrasonic imaging.
2. Description of the Prior Art
A known linear scan type ultrasonic imaging system which can enlarge and contract an image emits ultrasonic beams from an array type probe for parallel run scanning of an object under examination. Echo signals are converted to digital signals for processing. Data from the digital signals of individual ultrasonic rasters corresponding to a given ultrasonic rate are stored in respective line memories of a line memory group. Then the data stored in line memories are stored in a frame memory for each frame. When reading out from the frame memory, the enlargement or contraction of the image is effected by using a read zoom operation. The readout data are then converted into analog signals, which are in turn converted into video signals for display on a television monitor.
However, some problems arise when an image which has been enlarged or contracted is obtained. These problems will be explained with reference to prior art FIGS. 1-4.
Referring first to FIG. 1 (prior art) there is shown a probe 2 having a linear array of oscillators 3. Oscillators 3 are linearly scanned by a main unit 1 of an ultrasonic imaging system. Certain combinations of oscillators 3 in probe 2 are selectively driven for oscillation at a given ultrasonic rate, whereby ultrasonic beams are transmitted from probe 2 in the form of ultrasonic rasters A to D at the ultrasoninc rate to an object for detection. When enlarging or contracting the image, the correspondence between the ultrasonic rasters and the pixels of the television display vary depending on the magnification factor in the manner shown in FIGS. 2(a)-2(d).
FIG. 2(a) shows the correspondence in a case in which the magnification factor is 1. In this case, the ultrasonic rasters and pixels for television display have a one-to-one correspondence with respect to each other. FIG. 2(b) shows the correspondence relation in a case where the magnification factor is 1.5. In this case, there is no perfect one-to-one correspondence between the ultrasonic rasters and pixels for television display. The ultrasonic raster A corresponds to the first pixel, so that ultrasonic data of the ultrasonic raster A is written for the first pixel. For the second and third pixel, however, there are no corresponding rasters, so no ultrasonic data is written for these pixels. For the fourth pixels, ultrasonic raster C is a corresponding raster, so the ultrasonic data of that raster is written for the fourth pixel, and so forth.
FIG. 2(c) shows the correspondence in a case in which the magnification factor is 2. In this case, ultrasonic raster A corresponds to the first pixel, so ultrasonic data of raster A is written for this pixel. For the second pixel, however, there is no corresponding raster, so no ultrasonic data is written for this cell. Ultrasonic raster B corresponds to the third pixel so its data is written for this pixel and so forth.
FIG. 2(d) shows the correspondence in a case in which the magnification factor is 0.75. In this case, there is no perfect one-to-one correspondence between the ultrasonic rasters and pixels for television display. Ultrasonic raster A corresponds to the first pixel so that its data is written for this pixel. For the second and third pixels, there are no corresponding ultrasonic rasters, so no ultrasonic data is written for these pixels. For the fourth pixel, data of ultrasonic raster E is written. For the fifth and sixth pixels, there are also no corresponding ultrasonic rasters, so no ultrasonic data are written in these pixels.
When there are ultrasonic rasters which are not in a one-to-one correspondence between ultrasonic rasters and pixels for television display or there are pixels for which no ultrasonic data can be written because there are no corresponding ultrasonic rasters, it has been an accepted practice to perform a read zoom operation in the manner shown in FIG. 3. An interpolation is carried out and data obtained by that interpolation is written into the pixel which has no one-to-one corresponding ultrasonic raster.
FIGS. 3(a) and (b) show two manners of read zoom operation at the magnification factor of 1.5. In the case of FIG. 3(a), data (2a+c)/3 is obtained from ultrasonic data a and c of ultrasonic rasters A and C, respectively, and is written for the second pixel. For the third pixel, data (a+2c)/3 is obtained and written. In the case of FIG. 3(b), the data of an ultrasonic raster A is written for the second and third pixels, and the data c of raster C is written for the fifth and sixth pixels.
When these interpolation operation read zooms are performed for television display, the tone characteristic of the displayed image is considerably different from the actual tone characteristic. That is, in the case of the data interpolation operation shown in FIG. 3(a), the tone characteristic, which is shown in FIG. 4(b), is different from the actual tone characteristic shown in FIG. 4(a). Also, in the case of the data interpolation shown in FIG. 3(b), the tone characteristic shown in FIG. 4(b) is different from the actual characteristic. Since the conventional manner has such problems, the image which has been enlarged or contracted is unsatisfactory.